1. Field Of The Invention
The invention relates to a platform for a bladed wheel of a turbomachine comprising a drum and blades of hammer-head attachment type, the foot of which is retained in a circumferential groove of the drum, having bearing and/or retaining faces cooperating with the drum.
The present invention relates in particular but in a non-limiting way to the moving bladed wheels of a low-pressure compressor of a turbomachine.
2. Description Of The Related Art
Normally, these bladed wheels comprise a series of blades, each forming a separate piece having several parts, these blades being fitted directly onto the drum.
Such a blade 20 is shown in FIG. 1, on its own, and in FIGS. 2 to 4 when fitted in a circumferential groove 31 of a drum 30.
In this case, the blade 20 of the prior art that can be seen in FIG. 1 comprises the following parts:                a vane 21 extending essentially radially and axially and constituting the part that transmits the kinetic energy to the air stream,        a foot 22 constituting the part that maintains the blade on the rotor by being housed in the drum 30,        a platform 23 extending essentially axially and circumferentially, constituting the part that delimits and channels the air stream, and situated between the vane 21 and the foot 22, and        a radius 24 constituting the linking part between the vane 21 and the platform 23 and having an external progressive transition face with concave profile.        
After fitting in the circumferential groove 31 of the drum 30 of axis X-X′, as can be seen in FIGS. 2 to 4:                the platforms 23 abut against each other in pairs by their circumferential end face 23a (see FIG. 4), hence the blades 20 are immobilized in the circumferential direction relative to the drum 30 thanks to a lock (not represented) fitted between the row of blades 20 and in a locking notch (not represented) of the circumferential groove 31, and        the foot 22 and the circumferential groove 31 have opposing bearing faces 22a, 22b and 31a, 31b (see FIG. 3) which cooperate with each other in pairs, hence the blades 20 are immobilized radially and axially relative to the drum 30 (in rotation and under the effect of the centrifugal force, the inclined bearing faces 22a and 31a come into contact).        
In this case, an annular catch 25 is arranged on the internal face of the platform 23, on the downstream end (see FIGS. 1, 3 and 4) or on the upstream end of the platform 23. As a variant, both the downstream end and the upstream end of the platform 23 are equipped with a catch 25.
This catch 25 forms an abutment beneath the platform 23 which guarantees, when the motor is stopped and the blades 20 are not centrifuged, that the blade 20 bearing this catch remains in the correct position. In practice, the catch 25 forms by its free end a retaining face bearing against the top face (external face) of one of the lateral walls surrounding the circumferential groove 31: thus, the pivoting of the blade 20 about the foot 22 (about an axis parallel to the perpendicular direction in FIG. 3) is prevented. This catch 25 also serves to ensure the seal-tightness, because it also serves as a bearing surface for the O-ring seal 40 which is housed in an annular ridge 32 of the external face of the drum 30, close to the circumferential groove 31 (see FIG. 3), in order to prevent the recirculation of air below the blade 20.
In the present text, the terms “internal” and “external” designate radial positions respectively close to the axis X-X′ and distant from the axis X-X′ and the terms “upstream” and “downstream” designate axial positions along the axis X-X′, in the direction of the flow of the air stream, which are arranged respectively one in front of the other.
This type of one-piece blade 20 with hammer-head attachment is generally made of a titanium alloy with the application of several technologies: the parts exposed to the air stream, namely the vane 21, the radius 24 and the top face of the platform 23, the result of forging, whereas the other parts, namely the bottom face of the platform 23 and the foot, are obtained on completion of a post-forging machining step.
Because of the one-piece nature of these blades 20, it is not possible to make the different parts of the blade 20 using the methods and materials best suited to the geometrical and operating stresses of each of these parts.
Also known is how to produce blades that comprise a reduced blade platform (reduced circumferential extent) and which are pieces separate from other pieces consisting of individualized inter-blade platforms.
The documents U.S. Pat. No. 6,632,070, US 2007 0020102 and in particular document US 2006 0222502 relate to such a situation.
Thus, in the case of the document US 2006 0222502, a blade, an inter-blade platform, a blade, an inter-blade platform and so on are fitted alternately, in the circumferential groove of the drum. This way, on the one hand the blades with reduced platform and on the other hand the individualized inter-blade platforms can be manufactured separately, and therefore using different techniques and/or materials.
However, because of the larger number of pieces constituting the bladed wheel (practically twice as many), there are more handling operations when assembling (or disassembling) the bladed wheel. Since the play in the circumferential direction must be as small as possible (play to be taken up), the assembly of a larger number of pieces generates longer times and therefore an excess cost, as well as greater risks of error concerning the positioning of the pieces relative to each other and along the series of blades and inter-blade platforms.
Furthermore, with individualized inter-blade platforms, the locking system is more complex because the space generally occupied by the locks is used by the inter-blade platforms, which leads to the use of more complex inter-blade platforms called locking inter-blade platforms.
Furthermore, in this case, there is a risk of generating more air leaks, that is recirculations from the duct delimited by the external face of the platforms to the space situated under the platforms, because of the larger possible number of air passages between the reduced platform of a blade and the adjacent individualized inter-blade platform. To optimize the seal-tightness with individualized inter-blade platforms, it has been proposed to modify the geometry of the blade (or of the inter-blade platform), but these attempts have culminated in very complicated geometries, and would be the cause of major excess costs.